Improve ReportExportError to better include code context and location

[ColonelThirtyTwo]

• Remove ReportExportError.at_error and do it in ReportExportError.__str__, which is harder to miss.
• Add ReportExportError.code_context, holding the line near where an error occurs, and use it in __str__.
• map_jinja2_render_errors adds its location to a thrown ReportExportError if it doesn't have one.

This should further help narrow down Jinja template issues by providing more context.